This invention relates to a projection exposure apparatus used for exposing a pattern on a mask onto a photosensitive substrate in a photolithographic process for manufacturing a semiconductor device, liquid crystal display device, image pick-up device (CCD), thin-film magnetic head, and the like, and more particularly to a projection exposure apparatus having a mechanism for correcting the imagery characteristic of the projection lens system.
For manufacturing a semiconductor device or the like, a projection exposure apparatus is generally used, which transfers a pattern image formed on a reticle (serving as a mask) onto a wafer (or glass plate) as a photosensitive substrate through a projection lens system. Conventionally, an exposure apparatus of a collective exposure type, such as a stepper, has been used. However, a so-called step-and-scan type projection exposure apparatus has been recently substituted for the collective exposure type projection exposure apparatus. The step-and-scan type exposure apparatus exposes a pattern image onto a shot area on the wafer, while scanning both the reticle and wafer with respect to the projection lens system.
The projection lens system used in a projection exposure apparatus requires high resolution over substantially the entire exposure area, because the circuit pattern of the reticle must be precisely transferred onto the wafer. To this end, measures have been proposed to correct aberration in the projection lens system in every stage of the design and manufacturing processes. However, because the imagery characteristic of the projection lens system easily varies in response to the changes in atmospheric pressure, environmental temperature, absorption of illumination, etc., merely satisfying a certain imagery characteristic under a specific environmental condition is insufficient in practical use.
Recent projection exposure apparatus are equipped with an imagery characteristic correction mechanism, which measures the fluctuation in parameters of environmental conditions to calculate the changing amount of the imagery characteristic, or alternatively, directly measures the changing amount of the imagery characteristic and corrects the imagery characteristic of the projection lens system. The imagery characteristic of the projection lens system may be intentionally changed so that the apparatus matches with the characteristics of other projection exposure apparatus or photosensitizers.
Examples of the technique for correcting the imagery characteristic using an imagery characteristic correction mechanism include a method for driving the optical elements (lenses) in the projection lens system or the reticle along the optical axis of the projection lens system to correct the projection magnification, isotropic distortion (barrel distortion), spherical distortion, image plane distortion, etc. Another technique is to tilt the lens element of the projection lens system or the reticle with respect to a plane perpendicular to the optical axis of the projection lens system to correct anisotropic distortion (trapezoidal distortion) and image plane inclination. Still another technique is to seal the gap between certain lenses of the projection lens system and change the internal pressure of the sealed space to adjust the refractive index of the internal air to correct the projection magnification, isotropic distortion (barrel distortion), spherical aberration and image plane distortion.
When using such a conventional imagery characteristic correction mechanism, the imagery characteristic of the projection optical system is appropriately corrected; however, unintended positional shifts of the image-forming position may occur. This is because when driving the driven object (particularly lenses within the projection lens system, or the reticle) along the optical axis of the projection lens system, the driven object often slightly slips out of the optical axis and advances obliquely because it is difficult to drive the lens or reticle strictly parallel to the optical axis. Generally, when driving the lens or reticle in the optical-axis direction, the position of the lens or the reticle is strictly controlled by, for example, a position sensor. On the contrary, with respect to the direction perpendicular to the optical axis, the movement of the driven object is simply guided by a guide mechanism because it is typically not necessary to control the movement of the lens in the direction perpendicular to the optical axis. However, slackness (vibration) or elastic deformation of the guide mechanism may cause the driven object to move slightly out of line with the optical axis, which may result in displacement of the image-forming position of the pattern image off the optical axis.
During a semiconductor manufacturing process, multiple layers of different circuit patterns are exposed onto the wafer. Each pattern must be precisely superimposed on the previous pattern formed through the previous exposure. The projection exposure apparatus generally has an alignment sensor for detecting a registration mark formed on the previous pattern to determine a proper exposure position. Examples of the alignment sensor include a TTR (through-the-reticle) sensor, which monitors both the alignment mark on the reticle (referred to as a reticle mark) and the alignment mark on the wafer (referred to as a wafer mark) simultaneously. While a TTR sensor is very precise, there are several limitations in its operation because of the simultaneous measurement of the reticle and the wafer. To this end, an off-axis method is often used, in which only the wafer mark is detected using an alignment sensor fixed to the side of the projection lens system. In the off-axis method, the positional relation (base-line amount) between the reticle mark (more precisely, the center of the projected pattern image of the reticle) and the detection center of the alignment sensor is obtained and stored in advance. When the alignment sensor detects the position of the wafer mark, displacement of the wafer during exposure is then determined based on the detection result of the sensor and the prestored positional relation.
When exposing multi-layers of circuit patterns on the photosensitive substrate, the mark formed on the photosensitive substrate is aligned with the mark-detection optical system. The substrate stage is then moved from this position by the base-line amount to execute exposure. In this manner, the reticle pattern image is aligned with the circuit pattern, which has already been formed on the photosensitive substrate.
In a conventional projection exposure apparatus, the positional relationship between the image position of the reticle mark projected on the substrate stage and the detection center of the mark-detection optical system must be accurately detected. The absolute position of the reticle in a projection exposure apparatus is not so strictly regulated, as long as the reticle position relative to the photosensitive substrate is precisely controlled. For example, the reticle position relative to the projection optical system is not strictly controlled as long as the reticle is positioned within the exposure area and the precision of the projection optical system is assured in that area. A projection optical system is generally composed of a plurality of (e.g., twenty or more) lens elements. The optical axis of the projection optical system is defined by a composition of offset components of the center axes of the respective lens elements. It is not defined by the outer diameter of the projection optical system or the positions of the lens elements. If the area of reticle positions relative to the projection optical system is too large, the exposure area of the projection optical system must also be set large, which increases the cost.
In view of the circumstance described above, the position of the reticle is conventionally adjusted with reference to the outer diameter of the lens barrel of the projection optical system at a mechanical precision of about 200-400 .mu.m.
If the image-forming point changes through driving the imagery characteristic correction mechanism after the relation between the reticle mark and alignment sensor has been stored, then the wafer position slips out of the proper position, which causes an alignment error in superimposing pattern layers.
Occurrence of an alignment error is not limited to the case in which the lens element(s) or reticle are driven along the optical axis. For example, in the method in which the gap between the lens elements in the projection lens system is sealed to change the internal pressure to adjust the refractive index, the retainer supporting the lens elements may elastically deform due to the internal pressure, which causes the lens element to slightly move on or out of the optical axis. If the lens element moves in the direction perpendicular to the optical axis, the image-forming position would slip out of the proper position.
In addition to such unintended change in the image-forming position as described above, there is also implicit error in the system. For example, when the driven object is tilted with respect to a plane perpendicular to the optical axis of the projection lens system, anisotropic distortion may change, and at the same time, the entire image may shift. If, after distortion is corrected by tilting the driven object, the driven object is further driven along the optical axis to correct the isotropic distortion, then the image-forming position slightly changes even if the driven object is moved precisely along the optical-axis direction, because the driven object is already tilted.
Thus, when the imagery characteristic is corrected by driving the imagery characteristic correction mechanism, the image-forming position changes due to the imperfection of the driving mechanism, resulting in alignment errors.
Moreover, as the circuit patterns become smaller and more detailed, the requirement for alignment precision becomes stricter. To this end, in a recent technique, the magnification of the projection optical system is adjusted to correct the distortion of the photosensitive substrate. The position of the detection mark on the photosensitive substrate is detected to determine an amount of expansion (or contraction) of the photosensitive substrate. The magnification of the projection optical system is adjusted to the optimum value taking into consideration the expansion (or contraction) of the photosensitive substrate, so that exposure is performed under the optimum magnification, correcting the thermal distortion of the photosensitive substrate due to the high-temperature process.
When the magnification of the projection optical system is slightly changed through magnification adjustment, however, the center of the reticle pattern image also slightly shifts because the center of the reticle is not in precise alignment with the optical axis of the projection optical system in the conventional apparatus. When the reticle center is not coincident with the optical axis of the projection optical system, it is also offset from the alignment sensor within the projection exposure apparatus. Even though the magnification is set to the optimum state through the adjustment operation, the pattern image, which is to be superimposed onto the previous circuit pattern layer on the substrate, shifts as a whole.
As shown in FIG. 8, the center RCT of the reticle pattern image is offset from the optical axis AX of the projection optical system (in the effective area 201). In this example, the projection magnification is adjusted according to expansion of the photosensitive substrate, and the initial reticle image 202 is enlarged to the reticle image 203. The reticle image expands apart from the optical axis AX, which is a fixed reference axis, in proportion to the magnification. The vertices of the reticle image move outward along the dashed lines 204, 205 and 206, and the center RCT of the reticle image also slightly moves along the dashed line 204 because the center RCT of the reticle image is initially offset from the optical axis. This positional shift results in an alignment error.
When the position of the reticle is adjusted mechanically with reference to the outer diameter of the projection optical system, the center of the reticle pattern image generally shifts about 200-400 .mu.m from the optical center axis. If the magnification is changed by 20 ppm in this state, the center of the reticle pattern image shifts 4-8 nm. Alignment accuracy in recent projection exposure apparatus requires that the error be within 100 nm. Considering various other factors of alignment errors, such as imagery characteristic (mainly distortion) of the projection optical system, variation in the base-line amount, fluctuation in the stage control accuracy, or measurement error in the alignment sensor (mark detection optical system), it is not acceptable that the error caused by magnification adjustment occupies almost one tenth of the entire acceptable error range.
As a magnification adjusting mechanism, a mechanism for driving a part of the lens elements of the projection optical system along the optical axis is known. It is very difficult, however, to accurately drive the lens elements along the optical axis. A lens element slightly slips out of alignment with the optical axis in the direction perpendicular to the optical axis, or tilts with respect to the optical axis, which causes the reticle pattern image to shift. In this situation, positional shift of the reticle pattern image due to offset of the lens elements is further added to the divergence due to offset of the reticle pattern image center from the optical center axis, and the alignment error caused by magnification adjustment becomes still worse.